The present invention relates to optical communications and, more particularly, to a novel method of multiplexing optical signals onto a single optical fiber. Specifically, the method includes sampling multiple signals and coupling the sampled signals onto a single optical fiber. The optical multiplexing method, of the present invention, useful for both digital and RF analog signals, is applicable and for upgrading existing fiber optic communications networks and in new installations.
Multiplexing in optical fiber communications systems may be achieved by several existing technologies. The choice of multiplexing technology has a tremendous impact on the cost, performance and the upgradeability of the network. One multiplexing technology is wavelength division multiplexing (WDM). In WDM, data signals are each modulated on a separate wavelength of light and then coupled onto a single optical fiber. At the remote end of the optical fiber, the different wavelengths are optically separated and individually detected. WDM systems consequently require a large number of laser sources and detectors for each different wavelength.
An alternative to WDM that achieves high data rates on a single wavelength channel is known as optical time division multiplexing (OTDM). A typical OTDM system as shown in the diagram of FIG. 1 includes a single high speed pulsed laser 101. An optical splitter 103 splits the pulsed laser output. An external modulator 109 is used to modulate each signal onto an optical carrier. Optical delay lines 105, such as fibers of varying lengths, are used to inter-leave the pulsed signals of the different channels. The optical signals are then optically multiplexed by a combiner 111 onto a single optical fiber.
During the lifetime of fiber optic communications networks, there are often situations in which additional fiber is required but not available and upgradeability options are limited. Therefore, a method to easily upgrade a fiber optic communications network installed in the field would be advantageous. Current multiplexing technologies are not readily upgraded once they are installed. For instance, a fiber optic communications network that does not include WDM lasers installed cannot be upgraded using WDM without replacing all the laser transmitters and installing wavelength demultiplexers and/or filters at the receiver end. Upgrading a prior art OTDM network would require the installation of optical delay lines of precise delay values, a difficult procedure performed in the field.
There is thus a need for, and it would be highly advantageous to have a method of optical time division multiplexing by signal sampling which can be used to free optical fibers in an existing network that is straightforward to install and requires replacing a minimum number of existing network components.